Magnetic-fluid seals have been used to provide a positive hermetic barrier against gas and other contaminants. For example, a multiple-stage magnetic-fluid-seal apparatus is disclosed in U.S. Pat. No. 3,620,584, hereby incorporated by reference. A magnetic fluid, known as ferrofluid, used in such seals is described, for example, in U.S. Pat. No. 3,917,538, also hereby incorporated by reference. Typical magnetic-fluid exclusion seals operate through the employment of ferrofluid in a gap between rotary and stationary surfaces in the presence of a magnet, which provide focused radial magnetic-flux lines which retain the ferrofluid in a liquid O-ring in the gap about a shaft surface.
Ferrofluid-exclusion seals have been employed in computer magnetic-disc storage units and prevent contaminants from reaching the magnetic-disc area. Ferrofluid-exclusion-seal modules encased in a nonmagnetic housing have been used with computer-disc drives, to prevent the ingression of airborne or other particulates and vapors up and around the disc-drive shaft. The exclusion-seal module comprises a permanent ring magnet polarized axially, and generally a pair of magnetically permeable pole elements which sandwich the magnet, so that the inner peripheral ends of the pole pieces extend toward and form a close, noncontacting gap with the exterior shaft surface of the computer-disc drive.
In the past, ferrofluid was painted or otherwise applied in the gaps, to form a sealing liquid O-ring which was retained in position through the complete magnetic-flux circuit with the magnetically permeable disc-drive shaft. The exclusion-seal module is magnetically isolated from the disc memory, to prevent the low-strength magnetic field of the module from interfering with the information stored on the memory disc. While such ferrofluid-exclusion seals have been employed successfully, it has been found that the application of the ferrofluid, to form the liquid O-ring seal, requires a certain amount of skill, and, in addition, it has been difficult to determine when the correct amount of ferrofluid has been applied.